The sum() action of the spreadsheet app faces problems

I entered two 5 and used the sum action and it showed 10. Then on the next column, I entered two 12 and used sum again. It showed 24. When after that I right clicked on 24 and chose sum again for that cell, it showed that error!

I'd tested the app in many ways (including this one) with both Release and Debug modes. Everything worked well. I don't know what has happened for the app now!! :(

For sum():
I've declared and defined the function sum() in the spreadsheet.h and .cpp files. Below is its implementation:

@tomy
Nope, its not related to the age of the book and it will look the same also later on.
Its how widget application looks. unless you make your own style.
It looks like all the other win 7 apps which is by design.

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@tomy A Qt app usually looks like any other app on the system where it is executed. On Windows 7 it looks like Windows 7 applications, on Windows 10 it looks like a Windows 10 application. Not sure why you bother about this while reading the book.
If you want your app to look differently then you can use stylesheets: http://doc.qt.io/qt-4.8/stylesheet-examples.html